Off-axis illumination in an annular configuration has increased resolution and depth of focus for photolithographic projection imagers imaging a mask or reticle having dense lines and spaces and dense contacts. This is explained in a paper entitled "0.25 .mu.m Lithography Using a Deep-UV Stepper with Annular Illumination", authored by A. Yen, W. N. Partlo, R. W. McCleary, and M. C. Tipton, and presented at Microcircuit Engineering 92, Erlangen, Germany, in June of 1992, and will be published in "Proceedings of Microcircuit Engineering 92", Erlangen, Germany The advantage of annular illumination involves the diffractive effects of a reticle having dense lines and spaces and dense contacts, and an annular profile of illumination intensity is especially advantageous when imaging a reticle that has a region of densely packed features.
The simplest way to provide annular illumination of a mask or reticle in a photolithographic projection imager has been with a mask arranged at the pupil of the illuminator to block out the central region of the illumination and leave an annular configuration of illumination intensity extending around the masked out central portion. This has the serious disadvantage of reducing the total illumination energy available and thereby slowing down the imaging process.